Reverse blocking semiconductor device and a method for manufacturing the same

ABSTRACT

A reverse blocking semiconductor device that shows no adverse effect of an isolation region on reverse recovery peak current, that has a breakdown withstanding structure exhibiting satisfactory soft recovery, that suppresses aggravation of reverse leakage current, which essentially accompanies a conventional reverse blocking IGBT, and that retains satisfactorily low on-state voltage is disclosed. The device includes a MOS gate structure formed on a n− drift layer, the MOS gate structure including a p+ base layer formed in a front surface region of the drift layer, an n+ emitter region formed in a surface region of the base layer, a gate insulation film covering a surface area of the base layer between the emitter region and the drift layer, and a gate electrode formed on the gate insulation film. An emitter electrode is in contact with both the emitter region and the base layer of the MOS gate structure. A p+ isolation region surrounds the MOS gate structure through the drift layer and extends across whole thickness of the drift layer. A p+ collector layer is formed on a rear surface of the drift layer and connects to a rear side of the isolation region. A distance W is greater than a thickness d, in which the distance W is a distance from an outermost position of a portion of the emitter electrode, the portion being in contact with the base layer, to an innermost position of the isolation region, and the thickness d is a dimension in a depth direction of the drift layer.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. application Ser. No. 10/822,643 filed onApr. 12, 2004 now U.S. Pat. No. 7,049,674.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to power semiconductor devices for use inpower converters, in particular to IGBTs that are made using an FZ(floating zone) wafer and have bidirectional withstand capability,called bidirectional IGBTs or reverse blocking IGBTs.

B. Description of the Related Art

Conventional IGBTs (insulated gate bipolar transistors) that have aplanar pn junction structure are used with a dc (direct current) powersupply in their main application field of inverter circuits or choppercircuits. Since no problems occur in such application fields as long asa forward breakdown voltage is secured, obtaining reverse withstandcapability has not been considered an important factor in designing andmanufacturing such IGBTs.

In recent years, however, matrix converters such as a directly linkedconversion circuit are being employed in semiconductor converter systemsto execute an ac (alternating current) to ac conversion, an ac to dcconversion, or a dc to ac conversion. The use of bidirectional switchingelements in the matrix converter for the purpose of miniaturization,reduction of weight, high efficiency, fast response, and low cost of thecircuit has been studied. Therefore, IGBTs having reverse blockingcapability are required in order to obtain a bidirectional switchingelement consisting of anti-parallel connected reverse blocking IGBTs.

FIGS. 25( a), (b), and (c) show a matrix converter circuit. FIG. 25( a)shows the circuit including the switching elements for three phases.FIG. 25( b) shows one switching element using ordinary IGBTs, while FIG.25( c) shows a switching element using bidirectional IGBTs havingbidirectional withstand capability. Conventional IGBTs 1 a, 1 b in theconverter circuit as shown in FIG. 25( b) need diodes 2 a, 2 bseries-connected and in the forward direction connecting to the IGBTs tosecure reverse breakdown voltage because the IGBTs were not designed andproduced for obtaining effective reverse blocking capability. Theseries-connected diodes generate large losses resulting in lowconversion efficiency of the converter. Large number of device elementscauses difficulties in achieving small size, light weight, and low costof the converter. Reverse blocking IGBTs 1 c and 1 d as shown in FIG.25( c) can eliminate the series-connected diodes.

FIGS. 24( a) and (b) are sectional views of an essential part of areverse blocking IGBT. FIG. 24( a) shows the cross section when areverse voltage is applied and FIG. 24( b) shows the cross section whena forward voltage is applied. In FIGS. 24( a) and (b), a deep p+ typeisolation region 11 is formed by diffusion from front and rear surfacesof an n type FZ wafer that serves as an n− drift layer 3. Then, MOS gatestructures are formed comprising a plurality of p+ base layersselectively formed in the front surface region of the n− drift , layer3, n+ emitter region 5 selectively formed in the surface region of eachof the p+ base layers 4, gate oxide films 6, gate electrodes 7, and anemitter electrode 8. After the formation of the MOS gate structure, athickness of the n− type drift layer 3 is reduced to about 100 μm in thecase of reverse withstand voltage of 600 V by removing the rear portionof the drift layer. After the thickness reduction, a p+ collector layer9 is formed by ion implantation from the rear surface and followingannealing. Thus produced IGBT device is surrounded by the heavily dopedp+ isolation region 11 around the side face of the device at the dicingposition 10. Consequently, a depletion layer 12 on application of areverse voltage only extends towards the vicinity of the pn junction atthe p+ collector layer 9 and the p+ isolation region 11 and does notappear at the side face of the device at the dicing position. Thus, anelectric field develops only on the front surface of the device.Therefore, a sufficient reverse breakdown voltage can be attained. (SeeJapanese Unexamined Patent Application Publication Nos. H07-307469,2001-185727, 2002-076017, and 2002-353454 and M. Takei et al.,Proceedings of 2001 International Symposium on Power SemiconductorDevices and ICs, 2001, Osaka, Japan, pages 413-416, “600 V-IGBT withReverse Blocking Capability”.) If a conventional IGBT that lacks this p+isolation region 11 is reversely biased with an emitter at a groundpotential and a collector at a negative potential, electric fieldconcentration occurs at a substrate end region of a p+ collector layer,resulting in increased leakage current and insufficient reversebreakdown voltage.

Antiparallel connection as in FIG. 25( c) of the devices of FIGS. 24( a)and (b) make it possible to control forward and reverse current and towithstand application of forward and reverse voltages. Thus, the deviceof FIGS. 24( a) and (b) can be operated as a bidirectional device.Application of such bidirectional devices to an ac to ac converterallows direct conversion from ac to ac. Size of a converter circuit isdrastically reduced as compared with a converter consisting of aconverter, a capacitor, and an inverter. Consequently, the cost issubstantially reduced. The bidirectional device operates as an IGBT anda free wheeling diode (FWD).

At the time of reverse recovery in the FWD operation, accumulated excesscarriers are swept out by a depletion layer extending from the collectorside. If the quantity of the carriers in the collector side is large,reverse recovery peak current becomes large, which is hard recoverybehavior. For a reverse blocking IGBT to use as a FWD, improvement ofthe reverse recovery performance is essential. A method for improvingthe reverse recovery performance is known in which a collector layer inthe rear side is formed by low temperature activation and with a lowconcentration (See Japanese Unexamined Patent Application PublicationNo. 2002-353454).

FIG. 26 is a sectional view showing a peripheral breakdown withstandingstructure of an IGBT. (See Japanese Unexamined Patent ApplicationPublication No. 2000-208768). Referring to FIG. 26, in a front surfaceregion of n− drift layer 23 formed are p+ base layer 24 of a MOS gatestructure, and p type field limit layers 25 and n type channel stopperlayer 22 that are parts of a peripheral breakdown withstandingstructure. Each field limit layer 25 is in contact with respective fieldlimit electrode 27, which extends over oxide films 26 between fieldlimit layers 25. Channel stopper layer 22 is in contact with channelstopper electrode 21, which extends in the direction toward emitterelectrode 28. P+ collector layer 29 is formed in another surface regionof n− drift layer 23.

A peripheral breakdown withstanding structure of usual IGBTs and FWDs isconstructed so that the breakdown voltage is higher at the forward biasin which a collector electrode is at a positive potential and an emitterelectrode is at a negative potential. Specific breakdown withstandingstructures known in the art include a field limit layer, a field limitelectrode, a combination of a field limit layer and a field limitelectrode, SIPOS, and RESURF. A structure of the combination of a fieldlimit and a field limit electrode is disclosed in Japanese UnexaminedPatent Application Publication No. 2000-208768. The structure isadvantageous to obtain stable long term reliability. Generally, in ahigh humidity environment, when negative ions enter the surface regionof the oxide film of the breakdown withstanding structure, positivecharges are induced on the semiconductor surface beneath the oxide film,causing a lack of uniformity in electric field distribution, therebydecreasing breakdown voltage. The structure of the Japanese UnexaminedPatent Application Publication No. 2000-208768 facilitates a structurethat has relatively narrow distances between the field limit layers andrelatively long field limit electrodes in the region near the principaljunction, which is a pn junction between the n− drift layer and the player in contact with the emitter electrode. The structure decreasesopenings between the field limit electrodes where the oxide film isexposed and inhibits invasion of the negative ions. Therefore, theadverse effect of the negative ions can be prevented.

However, distribution of equipotential lines in such a combination offield limit layer 25 and field limit electrode 27 is sensitive to thearrangement of lengths, depths, and intervals of the layers and theelectrodes. For uniform distribution of electric potential and electricfield strength shared by each field limit layer 25, generally thedistances between field limit layers 25 must be made relatively narrowin the side of emitter electrode 28 and gradually widen towards theperiphery of the element. The distances between field limit layers 25 inthe region nearest to emitter electrode 28, in particular, are such thatjoining of built-in depletion layers at zero volt bias occurs betweenthe adjacent p layers, which is the principal junction of the fieldlimit layer. The distance between outermost field limit layer 25 andchannel stopper layer 22 is set to be 162 μm for a 1,200 volt devicethat is about the diffusion length of minority carriers so that adepletion layer does not reach channel stopper layer 22. As a result,the length of the breakdown withstanding structure of the 1,200 V deviceis designed to be 708 μm to obtain a stable breakdown withstandingstructure with little effect of surface charges.

A resistive film also has been used to achieve forward and reverseblocking characteristics. The technique uses a resistive nitride filmformed on the oxide film in the breakdown withstanding structure. Aminute amount of electric current flows in the resistive nitride film toattain uniform electric potential distribution and to enhance abreakdown voltage. This technique can be used in both forward andreverse directions in a reverse blocking IGBT in particular, eliminatinga field limit layer and a field plate electrode. Consequently, a lengthof a breakdown withstanding structure comprising a resistive film can bemade shorter than that of the field limit structure comprising a fieldlimit layer and a field plate electrode. Unfortunately, a THB test(temperature humidity biased test), which is a kind of long termreliability test, demonstrated degradation of reverse blockingcapability. The THB test examined long term variation of reverseblocking capability by placing a reverse blocking IGBT module in a hightemperature and humidity environment of 85% RH and 125° C. and applyingreverse bias voltage of 80% of the rated voltage. This degradation canbe attributed to the resistive property of the nitride film that causescorrosion in the environment. The corrosion makes electric potentialdistribution not uniform causing an electric field concentration thatdegrades the blocking performance. Therefore, it is urgently required todevise a breakdown withstanding structure of a reverse blocking IGBTthat performs satisfactory long term reliability.

It has been revealed that diode operation performance of the IGBTdisclosed in Japanese Unexamined Patent Application Publication No.2002-353454 cited above is not improved even if the rear collector layeris made low injection because holes are also injected from the heavilydoped p+ isolation region in the diode operation. Therefore, a structureis needed that suppresses hole injection from the p+ isolation layer.

Moreover, leakage current in the reverse biased condition, in which theemitter is positive and the collector is negative as shown in FIG. 24(a), depends on emitter injection efficiency, which is a parameter todetermine an open base amplification factor of the pnp transistor. Theemitter injection efficiency is substantially determined by a p+ layer(not shown in FIGS. 24( a) and (b)) formed in the surface region in thep+ base layer between n+ emitter regions 5,5, the p+ layer being incontact with the emitter electrode. The p+ layer is deeper than n+emitter region 5 and shallower than p+ base layer 4, and is doped moreheavily than p+ base layer 4. Since the p+ layer is extremely heavilydoped, in an amount which can be more than 1×10¹⁹ cm⁻³, in order toprevent latch-up, emitter injection efficiency may be higher than 0.9.As a result, the leakage current at high temperature is more than 10mA/cm², which is about 100 times greater than is typical. The emitterinjection efficiency can be decreased by forming an n+ layer doped moreheavily than n− drift layer 3 under p+ base layer 4. The n+ layer has adepth covering p+ base layer 4 in the case of a planar type, while then+ layer is disposed between p+ base layer 4 and n− drift layer 3 in thecase of a trench type. The n+ layer in the case of the planar type,however, produces a rigorous decrease in electric field intensity duringoff-operation, deteriorating blocking performance. Therefore, a means isneeded that reduces the reverse leakage current more readily.

Since thickness of an oxide film that is a diffusion mask for forming ap+ isolation region is not great enough according to conventionaltechnology, boron atoms eventually penetrate through the oxide film inhigh temperature diffusion around 1,250° C. forming a p+ layer evenunder the oxide film. This situation hinders formation of a normal MOSstructure and an abnormal chip of IGBT may be formed that will notturn-on.

IGBTs that have reverse blocking capability must withstand high voltagein a reverse biased condition of positive emitter electrode and negativecollector electrode as well as in a forward biased condition.Accordingly, a known reverse blocking IGBT comprises a structure forreverse blocking in which a p+ isolation region is formed surroundingthe end portion and spanning from front surface to rear surface of thedevice. An IGBT having that structure, however, failed to achievereverse breakdown voltage equivalent to forward breakdown voltage whenthe combination structure of field limit layers and field limitelectrodes described earlier is employed.

Measurement of breakdown voltage was made on an example of a reverseblocking IGBT with a rated voltage of 1,200 V by applying forward andreverse biased voltages, and resulted in a forward breakdown voltage of1,480 V that is satisfactory, but a reverse breakdown voltage of 1,220 Vthat affords an unacceptably small margin. This poor reverse blockingperformance resulted because a depletion layer reaches-through to theprincipal junction at a reverse bias of about 1,200 V and holes enterthe depletion layer generating leakage current through a path beneaththe breakdown withstanding structure corresponding to the bias voltage.

As such, in the reverse biased condition, reach-through of depletionlayer occurs in the region of the breakdown withstanding structure at asmaller voltage than the forward breakdown voltage. This causes a lowerreverse breakdown voltage than forward breakdown voltage. There are tworeasons for the reach-through of depletion layer in the reverse-biasedcondition. First, in the reverse-biased condition, two types ofdepletion layers develop: a depletion layer that develops verticallyfrom the pn junction at the rear collector layer towards the frontsurface, and a depletion layer that develops laterally from theperipheral isolation region towards the principal pn junction. Withincrease of the applied reverse voltage, the two depletion layers pinchoff and the number of electrons necessary for depletion of the driftlayer decreases with increase of the voltage. This situation tends toexpand the depletion layer, resulting in the above-describedreach-through at a lower voltage than the forward breakdown voltage.FIG. 27 shows this situation.

Secondly, some depletion layers are joined together at zero biascondition. Depletion layers are joined together in the region from theprincipal pn junction to a plurality of field limit layers already atzero bias condition. When the depletion layers expand from the rear sideand from the isolation layer in a reverse biased condition, thedepletion layer reaches-through towards the principal junction at thetime when the depletion layer from the rear face and the isolation layerarrive at the field limit layer to which the depletion layer is alreadyexpanded at zero bias.

Therefore, the reach-through of depletion layer to the principaljunction around the emitter must be prevented in the reverse biasedcondition, and a structure is needed thereby achieving stable long termreliability.

The present invention is directed to overcoming or at least reducing theeffects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

It is an object of the present invention to prevent the depletion layerfrom reach-through to the principal junction around the emitter sideeven in a reverse-biased condition. By achieving this object the aboveproblems are overcome and a breakdown withstanding structure of an IGBTwith stable long term reliability is provided.

It is a further object of the invention to provide such a reverseblocking semiconductor device that shows no adverse effect of anisolation region on reverse recovery peak current, that has a breakdownwithstanding structure exhibiting satisfactory soft recovery, thatsuppresses aggravation of reverse leakage current that essentiallyaccompanies a conventional IGBT, and that retains satisfactorily lowon-state voltage.

These and other objects according to the invention are provided by areverse blocking semiconductor device and a manufacturing methodtherefore. A reverse blocking semiconductor device of the inventioncomprises a drift layer of a first conductivity type; a MOS gatestructure including a base layer of a second conductivity typeselectively formed in a front surface region of the drift layer, anemitter region of the first conductivity type selectively formed in asurface region of the base layer, a gate insulation film covering asurface area of the base layer between the emitter region and the driftlayer, and a gate electrode formed on the gate insulation film; anemitter electrode in contact with both the emitter region and the baselayer; an isolation region of the second conductivity type surroundingthe MOS gate structure through the drift layer and extending acrosswhole thickness of the drift layer; a collector layer of the secondconductivity type formed on a rear surface of the drift layer andconnecting to a rear side of the isolation region; and a collectorelectrode in contact with the collector layer; wherein a distance W isgreater than a thickness d, in which the distance W is a distance froman outermost position of an portion of the emitter electrode, theportion being in contact with the base layer, to an innermost positionof the isolation region, and the thickness d is a dimension in a depthdirection of the drift layer.

In one preferred embodiment, the collector layer is formed on the rearsurface of the drift layer, a thickness of the drift layer having beenreduced.

In another preferred embodiment, lattice defects are introduced at leastin the base layer.

Preferably lattice defects are introduced homogeneously to whole thefront surface of the semiconductor device for a purpose of reducinglifetime of minority carriers in the semiconductor device.

The present invention also provides a method for manufacturing thisreverse blocking semiconductor device according to the presentinvention. The method comprises steps of preparing a substrate of afirst conductivity type; forming a MOS gate structure includingprocesses of selectively forming a base layer of a second conductivitytype in a front surface region of the substrate, selectively forming anemitter region of the first conductivity type in a surface region of thebase layer, forming a gate insulation film on the surface of the baselayer, the surface being between the emitter region and the frontsurface of the substrate without the emitter region, and forming a gateelectrode on the gate insulation film; forming an emitter electrode incontact with both the emitter region and the base region; selectivelyforming a peripheral region of the second conductivity type surroundingthe MOS gate structure through a portion of the substrate outside theMOS gate structure, a part of the peripheral region being to become anisolation region; removing a rear surface region of the substrate to apredetermined thickness to form the isolation region extending acrosswhole the thickness and to form a drift layer of the first conductivitytype inside the isolation region; forming a collector layer of thesecond conductivity type on a rear surface of the drift layer andconnecting to a rear side of the isolation region; and forming acollector electrode in contact with the collector layer. The steps tomanufacture the semiconductor device are conducted in such a manner thata distance W is greater than a thickness d, in which the distance W is adistance from an outermost position of a portion of the emitterelectrode, the portion being in contact with the base layer, to aninnermost position of the isolation region, and the thickness d is adimension in a depth direction of the drift layer. The step ofselectively forming the peripheral region to become an isolation regionis conducted by diffusing impurities using a diffusion mask of an oxidefilm formed on the front surface of the substrate, the oxide film havinga thickness Xox satisfying an inequality of the following Formula (1):

$\begin{matrix}{X_{ox} > {\sqrt{\frac{D_{ox}}{D_{S}}}X_{S}}} & {{Formula}\mspace{14mu}(1)}\end{matrix}$whereinD_(OX) is a diffusion coefficient of the impurity in the oxide film,D_(S) is a diffusion coefficient of the impurity in material of thesubstrate,X_(S) is a diffusion depth of the impurity in material of the substrate.

When holes are injected from a collector, the holes generally tend toflow in the shortest path. If the distance from the isolation region toa surface contact region of the emitter electrode, which is a distancein the breakdown withstanding structure region from the isolation regionto a so-called active region, is longer than the thickness of the n−drift layer, the holes tends to be injected from the collectorpositioned just under the active region and flow towards the emitterelectrode, rather than injected from the isolation region. As a result,hole injection from the isolation region towards the active regionrelatively decreases. Further, when the distance between the isolationregion and the active region is longer than an ambipolar diffusionlength of the minority carriers, that are holes, the concentration ofthe holes injected from the isolation region decays rapidly enough.Therefore, hole injection from the isolation region can be ignored inthis case.

Since introduction of lattice defects decreases lifetime, introductionof lattice defects at least in the p+ base layer 4 reduces emitterinjection efficiency. However, if the lattice defects are introducedlocally in the surface region only, loss tradeoff relationshipdeteriorates. Accordingly, the lattice defects are preferably introducedinto the whole device homogeneously in the depth direction and in theradial direction. Electron beam irradiation is preferably employed forsuch introduction of lattice defects. Because the collector isinherently made low injection, if the electron beam irradiation dose istoo large or acceleration voltage is too high, damage to the device issevere and the lifetime becomes too short, resulting in an increase inon-state voltage.

Accordingly, a preferred method for manufacturing a reverse blockingsemiconductor device according to the present invention comprises a stepof introducing lattice defects homogeneously to whole of the frontsurface of the semiconductor device in order to reduce the lifetime ofminority carriers in the semiconductor device. The step of introducingthe lattice defects is conducted by electron beam irradiation with anenergy less than 5 MeV and a dose less than 100 kGy. An energy less than5 MeV and an irradiation dose less than 100 kGy controls the increase inon-state voltage minimum and suppresses reverse leakage current.

Another preferred method for manufacturing a reverse blockingsemiconductor device according to the present invention comprises a stepof introducing lattice defects at least in the base layer by electronbeam irradiation with a dose in a range of 20 kGy to 60 kGy.

A reverse blocking semiconductor device according to the presentinvention comprises a drift layer of a first conductivity type; a MOSgate structure including a base layer of a second conductivity typeselectively formed in a front surface region of the drift layer, anemitter region of the first conductivity type selectively formed in asurface region of the base layer, a gate insulation film covering asurface area of the base layer between the emitter region and the driftlayer, and a gate electrode deposited on the gate insulation film; anemitter electrode in contact with both the emitter region and the baselayer; an isolation region of the second conductivity type surroundingthe MOS gate structure through the drift layer and extending across thewhole thickness of the drift layer; a collector layer of the secondconductivity type formed on a rear surface of the drift layer andconnecting to a rear side of the isolation region; a collector electrodein contact with the collector layer; a plurality of field limit layersof the second conductivity type in the front surface region of the driftlayer between the emitter electrode and the isolation region, each ofthe field limit layers having a ring shape; and a plurality of fieldlimit electrodes, each in contact with each of the field limit layers,having a ring shape, and at a floating electric potential; wherein aplurality of the field limit electrodes exist in the side of the emitterelectrode and each of the field limit electrodes in the side of theemitter electrode has a larger outward extension portion than an inwardextension portion; and a plurality of the field limit electrodes existin the side of the isolation region and each of the field limitelectrodes in the side of the isolation region has a larger inwardextension portion than an outward extension portion.

Preferably the reverse blocking semiconductor device of the inventionadditionally comprises at least one high concentration layer of thefirst conductivity type in at least a portion of one of the frontsurface region of the drift layer in the side of the emitter electrodeand the front surface region of the drift layer in the side of theisolation region, the high concentration layer containing higherconcentration of impurities than the drift layer.

Advantageously, a surface concentration of impurities in the highconcentration layer is less than 10¹⁷ cm⁻³.

It is preferred that a distance Wg between the adjacent field limitlayers is larger than 2 times the Wbi, the Wbi being a width of abuilt-in depletion layer extending from the field limit layer towardsthe drift layer in a condition in which the emitter electrode and thecollector electrode are at an equal electric potential.

Advantageously, WGi>1.6 Xj+2 Wbi in which WGi is a width of an insulatorfilm between (I-1)-th field limit layer and i-th field limit layer, Xjis a diffusion depth of the field limit layer, and Wbi is a width of abuilt-in depletion layer extending from the field limit layer towardsthe drift layer in a condition in which the emitter electrode and thecollector electrode are at an equal electric potential.

Preferably a thickness of the drift layer Wdrift satisfies theinequality in the following Formula (2):

$\begin{matrix}{{\sum\limits_{i = 1}^{n}\; L_{Ni}} \geq W_{drift}} & {{Formula}\mspace{14mu}(2)}\end{matrix}$whereinL _(Ni) =W _(Gi)−(1.6X _(j)+2W _(bi))in whichi is an order number of the field limit layer,W_(G)i is the width of an insulator film of oxide between (i-1)-th andi-th field limit layer,n is the total number of the field limit layers.Xj is a diffusion depth of the field limit layer, and Wbi is a width ofa built-in depletion layer extending from the field limit layer towardsthe drift layer in a condition in which the emitter electrode and thecollector electrode is at an equal electric potential.

Preferably, the sum Σ LNi and a sum Σ LOPi satisfies an inequality ΣLOPi/Σ LNi<0.7, in which LOPi is a distance between (i-1)-th field limitelectrode and i-th field limit layer.

In a preferred embodiment, a reverse blocking semiconductor device ofthe invention additionally comprises an intermediate field buffer regionof the second conductivity type in a surface region of the drift layerbetween the plurality of field limit electrodes in the side of theemitter electrode and the plurality of field limit electrodes in theside of the isolation region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIG. 1 is a schematic sectional view of an essential part of anembodiment of a reverse blocking semiconductor device according to thepresent invention.

FIG. 2 is a characteristic chart showing dependence of reverse recoverycurrent in diode operation on the distance W between an isolation regionand an active region in an embodiment of a reverse blocking IGBTaccording to the present invention.

FIG. 3 shows dependence of reverse leakage current RIces on electronbeam irradiation dose.

FIG. 4 shows distribution of equipotential lines when reverse biasvoltage of 800 V is applied to the 600 V reverse blocking IGBT of anembodiment according to the present invention.

FIG. 5 is a graph of complimentary error function.

FIG. 6 is a characteristic chart showing reverse recovery operation of areverse blocking IGBT according to the present invention.

FIG. 7 shows dependence of reverse leakage current RIces on electronbeam irradiation dose.

FIG. 8 shows dependence of on-state voltage on electron beam irradiationdose.

FIG. 9 is a perspective view of a breakdown withstanding structure of athird example of embodiment.

FIG. 10 is a sectional view of the breakdown withstanding structure ofthird example of embodiment; FIG. 10( a) is a sectional view of wholebreakdown withstanding structure, while FIGS. 10( b) through 10(d) arepartial sectional views of variations from the portion Y in FIG. 10( a).

FIGS. 11( a) and (b) are sectional views of the breakdown withstandingstructure illustrating development of depletion layer in a forwardbiased condition (FIG. 11( a)) and a reverse biased condition (FIG. 11(b)).

FIGS. 12( a) and (b) are partial sectional views showing expansion of adepletion layer between the field limit layers. FIG. 12( a) shows a casea width WG of the oxide film is wide, and FIG. 12( b) shows a case thewidth WG is narrow.

FIG. 13 is a characteristic chart illustrating relationship betweenreverse breakdown voltage and a sum of distances LNi that is a width ofa neutral region between the field limit layers at a zero bias conditionshown in FIG. 11.

FIG. 14 is a partial sectional view showing a width Lop of an openingbetween a field limit electrode 27 a and an adjacent field limit layer25.

FIGS. 15( a) through (d) are schematic partial sectional views of acomparative example illustrating reach-through of a depletion layer to aprincipal pn junction in the emitter side. FIG. 15( a) shows net doping,FIG. 15( b) shows electron concentration, FIG. 15( c) showsequipotential lines, and FIG. 15( d) shows hole current density.

FIGS. 16( a) through (d) are schematic partial sectional views of thebreakdown withstanding structure region with an opening width Lop of 7μm. FIGS. 16( a), (b), (c), and (d) show net doping, electronconcentration, equipotential lines, and hole current density,respectively.

FIGS. 17( a) and (b) are sectional views of an example of embodiment inwhich the n type high concentration layers are formed in both theemitter electrode side and the isolation region side in the breakdownwithstanding structure region. FIG. 17( a) shows a cross section of thebreakdown withstanding structure region, and FIG. 17( b) shows apartially enlarged section of the emitter electrode side.

FIG. 18 is a schematic sectional view of the breakdown withstandingstructure region of an example of embodiment in which an n type highconcentration layer is formed in the isolation region side of thebreakdown withstanding structure region.

FIG. 19 is a detailed sectional view of the breakdown withstandingstructure region before forming n type high concentration layers.

FIG. 20 shows schematically a partial cross section of the breakdownwithstanding structure region of an embodiment in which an n type highconcentration layer is formed in the isolation region side of thebreakdown withstanding structure region. FIGS. 20( a) through (d)illustrate net doping, electron concentration, equipotential lines, andhole current density, respectively.

FIG. 21 shows dependence of reverse breakdown voltage on phosphor doseto the high concentration layer of the structure of FIG. 18.

FIG. 22 is a partial sectional view of a breakdown withstandingstructure illustrating an n type high concentration layer formed in theemitter electrode side that is about half of the breakdown withstandingstructure region.

FIG. 23 is a characteristic chart showing variation of reverse breakdownvoltage in a long term in a THB test.

FIGS. 24( a) and (b) are sectional views of an essential part of aconventional reverse blocking IGBT.

FIGS. 25( a), (b), and (c) show a matrix converter circuit.

FIG. 26 is a sectional view illustrating a peripheral breakdownwithstanding structure of a conventional IGBT.

FIG. 27 is a sectional view illustrating a peripheral breakdownwithstanding structure of a conventional IGBT.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a reverse blocking semiconductor devicethat shows no adverse effect of an isolation region on reverse recoverypeak current, that has a breakdown withstanding structure exhibitingsatisfactorily soft recovery, and that suppresses aggravation ofrecovery leakage current that essentially accompanies a conventionalreverse blocking IGBT; and yet, on-state voltage is controlled within asatisfactorily low value. Consequently, a matrix converter can beconstructed with easy operation and low loss using an invented reverseblocking semiconductor device.

FIG. 1 is a sectional view of an essential part of a reverse blockingIGBT of an exemplary embodiment of the present invention illustratingthe relationship between a distance W between an isolation region and anactive region, and a thickness d in a depth direction of an n− driftlayer.

FIG. 2 is a characteristic chart showing dependence of reverse recoverycurrent in diode operation on the distance W in a reverse blocking IGBTfor 600 V breakdown voltage applying the teachings of the presentinvention. In FIG. 2, the abscissa represents the ratio W/d, where W isthe distance between the isolation region and the active region, and dis the thickness of the n− drift layer. More particularly, W is thedistance from an outermost position of an portion of the emitterelectrode, the portion being in contact with the p+ base layer 4, to aninnermost position of the p+ isolation region. The distance W isindicated in FIG. 1. The ordinate in FIG. 2 represents reverse recoverypeak current normalized by the value at W/d of four and at W of twotimes ambipolar diffusion length La. Here, applied voltage Vcc in thereverse recovery is 100 V. Structure of the IGBT is as follows.

An n− type FZ wafer is prepared having thickness of 525 μm and impurityconcentration of 1.5×10¹⁴ cm⁻³. An initial oxide film 1.6 μm thick isformed on the front surface of the wafer. A region with a width of 100μm is selectively etched on a peripheral portion of each device. A boronsource is applied to the front surface and heat treatment is conductedto perform boron deposition. After removing boron in the oxide film by aboron glass etching process, diffusion of the boron is performed to adepth of 120 μm at 1,200° C. in an oxygen atmosphere to form a p+isolation region 11. A MOS gate structure as in a common IGBT is formedin the front surface region. The MOS gate structure includes p+ baselayer 4, n+ emitter region 5, gate oxide film 6, gate electrode 7, andemitter electrode 8. After that, a rear surface of the wafer is groundto a thickness of 100 μm to form n− drift layer 3. The thickness isappropriately about 180 μm in the case of an IGBT with a breakdownvoltage of about 1,200 V. Then, ion implantation of boron is conductedwith a dose of 1×10^(13 cm) ⁻² in the rear surface and annealing at 350°C. for 1 hour is followed, to form p+ collector layer 9 with a peakconcentration of 1×10¹⁷ cm⁻³ and a thickness of about 1 μm. Finally, acollector electrode is formed.

Thus, a reverse blocking IGBT is produced. The collector layer may beactivated after the boron ion implantation to the rear surface byirradiating with a wholly solid-state YAG 2ω laser in the range of 500mJ/cm² to 4 J/cm². The distance W in this example is in a range of 80 μmto 400 μm.

When W/d is smaller than one, i.e., the distance W from the activeregion to the isolation region is less than the thickness d of the n−drift layer, the reverse recovery peak current rapidly increases. Withdecrease of the distance W to the isolation region, hole injection fromthe isolation region becomes dominant over injection from the collectorlayer. This is due to the fact that acceptor concentration in theisolation region is higher by more than two orders of magnitude thanthat in the p+ collector layer and the distance from the isolationregion to the emitter electrode is shorter than the drift layerthickness, resulting in lower resistance for hole injection from theisolation region. Consequently, carrier concentration is larger in therear side in the carrier distribution in the on-state of the IGBT.Reflecting the situation, reverse recovery peak current is relativelylarge in the case of W/d smaller than one.

In the case lifetime is decreased, the reverse recovery peak current isfurther reduced in comparison with the case of non-killer, in which thelifetime is not decreased. In FIG. 2, the ambipolar diffusion lengthLa1=194 μm in the case of non-killer, while the ambipolar diffusionlength La2=82 μm in the case an electron beam of 4 Mrad (=40 kGy) isirradiated. The thickness d of the n− drift layer is about 100 μm. Theelectron beam irradiation decreased the reverse recovery current.

Some specific examples of embodiment of the present invention will bedescribed in the following.

EXAMPLE 1

Reverse leakage current is generally larger than leakage current in ausual forward direction IGBT. This is because of the high dose of the p+layer in contact with the emitter electrode on the one hand, and becauseresidual lattice defects or damage at the time of low temperatureactivation of the collector layer on the other hand.

FIG. 3 shows dependence of reverse leakage current RIces on electronbeam irradiation dose. FIG. 4 shows distribution of equipotential lineswhen a reverse bias voltage of 800 V is applied to the 600 V reverseblocking IGBT. A zero volt line lies at about 30 μm from the frontsurface. The front surface side of the 0−V line is a neutral regionwithout depletion of electric charges. The p+ layer is usually formedwith a dose of more than 1×10¹⁹ atoms/cm³ to avoid latch-up as describedin background of the invention. The RIces is represented by thefollowing Formula (3)RI _(CES)=β(I _(gen—n) +I _(gen—p))+I _(diff)  Formula (3)where I_(gen n) and I_(gen p) are generation currents in the drift layerand the collector layer, respectively. I_(diff) is diffusion current ofminority carriers, the current being negligible at high temperature.

From the Formula (3), an emitter amplification factor β is,

$\begin{matrix}{\beta = {\frac{1}{1 - {\gamma\alpha}_{T}} \cong \frac{1}{1 - \alpha_{T}} \cong \frac{2D_{h}\tau_{p}}{W_{D}^{2}}}} & {{Formula}\mspace{14mu}(4)}\end{matrix}$Second order approximation is used to obtain the Formula (4). Emitterinjection efficiency γ is approximately one in a reverse biasedtransistor. τ_(p) represents lifetime of minority carriers, and Dhrepresents diffusion coefficient of holes in the drift layer. W_(D)represents a neutral region width in the drift layer, which is about 30μm in the case of FIG. 4. Hence, I_(gen n) is represented by Formula (5)below.

$\begin{matrix}{I_{gen\_ n} = \frac{{qn}_{i}{AW}}{2\tau_{sc}}} & {{Formula}\mspace{14mu}(5)}\end{matrix}$where A is an area of the active region, W: depletion layer width,τ_(sc): generation lifetime in the space charge region. Dominantcaptured level due to the electron bean irradiation is sufficientlyshallow and τ_(p) is shorter enough than τ_(sc). Therefore, the RIces issufficiently small.

The abscissa in FIG. 3 represents electron irradiation dose in Mrad (1Mrad=10 kGy), the ordinate represents reverse leakage current RIces.FIG. 3 illustrates leakage currents RIces for without bias at the gate(G-E short circuited) and RIce+ for application of G-E voltage of +15 Vat the gate in the case with a collector layer thermally activated at350° C. for 1 hour; and a leakage current RIce+ (Laser) for applicationof G-E voltage of +15 V at the gate in the case with the p+ collectorlayer laser activated. FIG. 3 shows that reverse leakage current islarger in the case with the gate and emitter short-circuited than thecases of a +15 V application. The application of +15 V at G-E forms aninversion layer (threshold value is 7.5 V) that short-circuits the n+emitter layer and the n− drift layer. Thus, parallel pin diodestructures are formed, thereby decreasing hole injection efficiency inthe front surface side. In actual converter operation, however, thereverse leakage current is desired to be small, even without biasvoltage on the gate. FIG. 3 shows that electron beam irradiation reducesthe reverse leakage current in no bias condition between G-E, and theleakage current is reduced to nearly the same value as the case of G-E+15 V application, by irradiation of the dose of 10 Mrad (=100 kGy).This result demonstrates an effect of reduction of amplification factorof the above-noted pnp transistor. FIG. 3 further shows that reverseleakage current is suppressed to less than ⅓ by laser irradiation thatfully recrystallized the region around the p+ collector layer. This isalso the above-noted effect of suppression of generation current in thep+ layer.

On-state voltage of the IGBT is 2.0 V for no electron beam irradiation,2.2 V for 10 Mrad irradiation, and 2.8 V for 20 Mrad irradiation. It hasbeen shown that on-state voltage enhancement can be controlled within10% for irradiation up to 10 Mrad.

Next the thickness of an SiO2 film for a mask for selectively formingthe heavily doped p+ isolation region will be described. As describedearlier, an oxide film is formed in an initial stage of themanufacturing process, and then selectively etched on the area forforming a p+ isolation region. Thickness of the oxide film necessary toform the isolation region can be calculated as follows.

Impurity concentration distribution under existence of a diffusionsource is given by the following Formula (6),

$\begin{matrix}{{N_{Si}\left( {x_{Si},t} \right)} = {N_{0}{erfc}\mspace{11mu}\left( \frac{x_{Si}}{2\sqrt{D_{Si}t}} \right)}} & {{Formula}\mspace{14mu}(6)}\end{matrix}$where N_(Si) is impurity concentration in silicon, N₀ is impurityconcentration at a surface position, x_(Si) is a distance from thesurface in the silicon, D_(Si) is a diffusion coefficient of boron inthe silicon, and t is diffusion time. The diffusion coefficient D_(Si)is represented by

$\begin{matrix}{D_{Si} = {D_{\infty}\exp\mspace{11mu}\left( {- \frac{E_{a}}{kT}} \right)}} & {{Formula}\mspace{14mu}(7)}\end{matrix}$where D∞ is a constant, Ea, an activation energy, k, Boltzmann constant,and T, absolute temperature. The activation energy Ea is about 3.7 eV.The diffusion coefficient is 1.0×10⁻¹¹ cm²/s at 1,300° C. Diffusiondepth required for a 600 V class reverse blocking IGBT is 120 μm. Inactual diffusion, the diffusion depth of 120 μm was attained with asurface impurity concentration of 1.2×10¹⁹ cm⁻³, diffusion temperatureof 1,300° C., and diffusion time of 83 hour. Letting the impurityconcentration N_(Si) in Formula (6) equal the doping concentration ofthe n type wafer, i.e., 1.5×10¹⁴ cm⁻³, which is a condition at the pnjunction, then N_(Si)/N₀ equals 1.25×10⁻⁵. Using the complimentary errorfunction (erfc) shown in FIG. 5, x_(Si)=104 μm is obtained. This valueapproximately agrees with the experimental value.

Boron diffusion in a thermal oxide film is similarly represented by

$\begin{matrix}{{N_{ox}\left( {x_{ox},t} \right)} = {N_{0}{erfc}\mspace{11mu}\left( \frac{x_{ox}}{2\sqrt{D_{ox}t}} \right)}} & {{Formula}\mspace{14mu}(8)}\end{matrix}$where x_(ox) is a distance from the surface of the oxide film, andD_(ox) is a diffusion coefficient of boron in the oxide film. Activationenergy of boron in the oxide film is about 3.5 eV. Diffusion coefficientat 1,300° C. is 1.29×10⁻¹⁵ cm²/s. A condition for the boron to passthrough the oxide film having thickness of 1.6 μm can be obtained asfollows. Letting x_(ox)=1.6 μm and N_(ox) equal the doping concentrationof the n type wafer, i.e., 1.5×10¹⁴ cm⁻³, then t=153 hour is obtainedusing FIG. 5. This means that an oxide film 1.6 μm thick allows maskingfor up to 150 hour. In actual diffusion, boron is drawn out from theoxide film to the silicon at the Si/SiO₂ interface due to largerdiffusion coefficient in the silicon side, thereby further reducingboron concentration at the silicon surface. According to processsimulation, the boron concentration is smaller by an order of magnitudethan the value calculated by Formula (8). This means that Formula (8)gives a safer estimate. Boron diffusion in silicon for t=153 hourresults in a diffusion depth of x_(Si)=141 μm according to Formula (6).The maximum depth of possible selective diffusion is 141 μm when a maskof oxide film having thickness of 1.6 μm is used. More generally, therelationship between diffusion time t_(d) and the thickness of an oxidefilm X_(ox) that is just passed through by boron is given by Formula (9)using Formula (8).

$\begin{matrix}{{N_{ox}\left( {X_{ox},t_{d}} \right)} = {N_{D} = {N_{0}{erfc}\mspace{11mu}\left( \frac{X_{ox}}{2\sqrt{D_{ox}t_{d}}} \right)}}} & {{Formula}\mspace{14mu}(9)}\end{matrix}$where N_(D) is doping concentration of the n type silicon. Letting adiffusion depth in silicon be X_(Si) at this time, following Formula(10) is derived using Formula (6).

$\begin{matrix}{{N_{Si}\left( {X_{Si},t_{d}} \right)} = {N_{D} = {N_{0}{erfc}\mspace{11mu}\left( \frac{X_{Si}}{2\sqrt{D_{Si}t_{d}}} \right)}}} & {{Formula}\mspace{14mu}(10)}\end{matrix}$

According to Formulas (9) and (10),

$\begin{matrix}{{N_{0}{erfc}\mspace{11mu}\left( \frac{X_{Si}}{2\sqrt{D_{Si}t_{d}}} \right)} = {N_{0}{erfc}\mspace{11mu}\left( \frac{X_{ox}}{2\sqrt{D_{ox}t_{d}}} \right)}} & {{Formula}\mspace{14mu}(11)}\end{matrix}$

Therefore, the following Formula (12) is derived.

$\begin{matrix}{\frac{X_{Si}}{X_{ox}} = {\sqrt{\frac{D_{Si}}{D_{ox}}} \approx {88\mspace{14mu}\left( {{at}\mspace{14mu} 1300^{{^\circ}}{C.}} \right)}}} & {{Formula}\mspace{20mu}(12)}\end{matrix}$

Thus, the maximum diffusion depth in silicon is determined by squareroot of a ratio of diffusion coefficient of boron in silicon todiffusion coefficient of boron in an oxide film for a fixed thickness ofa mask oxide film, and in no way depends on surface concentration ordiffusion time. Activation energy of boron diffusion is approximatelyequal in silicon and in an oxide film, and is about 3.5 eV.Consequently, the ratio of the diffusion coefficients is constant at anytemperature as implied by Formula (7). The above analysis concludes thatgiven a thickness of the mask oxide film, the maximum diffusion depth insilicon is uniquely determined. Necessary diffusion depth is 120 μm fora 600 V reverse blocking IGBT. According to Formula (12), the minimumnecessary thickness of oxide film is 1.36 μm. For a 1,200 V reverseblocking IGBT, a necessary diffusion depth is 200 μm and the minimumnecessary thickness of oxide film is 2.27 μm.

The analysis to this point has been based on the case of diffusion usinga diffusion source. Next, the case of driving-in is considered, in whichthe diffusion source is removed after deposition.

Distribution of boron concentration in an oxide film is represented bythe following Formula (13).

$\begin{matrix}{{N_{ox}^{\prime}\left( {x_{ox},t} \right)} = {\frac{Q_{ox}}{\sqrt{\pi\;{D_{ox}(t)}}}{\exp\left( {- \frac{x_{ox}^{2}}{4{D_{ox}(t)}}} \right)}}} & {{Formula}\mspace{20mu}(13)}\end{matrix}$where Q_(ox) is total number of impurities in the oxide film, andrepresented by the following expression using Formula (8), in whicht_(p) is deposition time.

$\begin{matrix}\begin{matrix}{Q_{ox} = {\int_{0}^{\infty}{{N\left( {x_{ox},t_{p}} \right)}\ {\mathbb{d}x}}}} \\{= {\frac{2}{\sqrt{\pi}}N_{0}\sqrt{D_{ox}t_{p}}}}\end{matrix} & {{Formula}\mspace{20mu}(14)}\end{matrix}$

Substituting Formula (14) in Formula (13), gives Formula (15):

$\begin{matrix}{{N_{ox}^{\prime}\left( {x_{ox},t} \right)} = {\frac{2N_{0}}{\pi}\sqrt{\frac{t_{p}}{t}}{\exp\left( {- \frac{x_{ox}^{2}}{4D_{ox}t}} \right)}}} & {{Formula}\mspace{20mu}(15)}\end{matrix}$

For diffusion in silicon, similarly,

$\begin{matrix}{{N_{Si}^{\prime}\left( {x_{Si},t} \right)} = {\frac{2N_{0}}{\pi}\sqrt{\frac{t_{p}}{t}}{\exp\left( {- \frac{x_{Si}^{2}}{4D_{Si}t}} \right)}}} & {{Formula}\mspace{20mu}(16)}\end{matrix}$

Provided an oxide film with a thickness X_(ox) is just passed through ata diffusion time t_(d) and a diffusion depth in silicon is X_(Si) atthat time,N _(Si)′(X _(Si) ,t _(d))=N _(ox)′(X _(ox) ,t _(d))  Formula (17)Therefore, the exactly same result as Formula (12) is obtained usingFormulae (15) and (16), i.e. the maximum diffusion depth is determinedby a thickness of a mask oxide film.

FIG. 6 is a characteristic chart showing reverse recovery operation of areverse blocking IGBT according to the present invention.

EXAMPLE 2

FIGS. 7 and 8 show characteristics of a second embodiment of theinvention that is different from that shown in Example 1. FIG. 7 showsthe relationship between electron beam dose and reverse leakage current,and FIG. 8 shows relationship between electron beam dose and on-statevoltage. This embodiment comprises p+ collector layer 9 with a peakconcentration of 1×10¹⁷ cm⁻³ and a thickness of about 1 μm that isformed by ion implantation of boron with a dose of 5×10¹³ cm⁻² on a rearsurface followed by annealing at 400° C. for 1 hour.

In FIG. 7, the abscissa represents electron beam irradiation dose inMrad (1 Mrad=10 kGy), and the ordinate represents reverse leakagecurrent RIces. Electron beam irradiation or helium irradiation isimplemented before grinding a rear surface of an FZ wafer for thepurpose of fast operation of a device. The electron beam irradiation canalso reduce reverse leakage current. Since the electron bean generateslattice defects homogeneously in the bulk of a device, transportefficiency in a reverse-biased condition is drastically reduced, therebydecreasing a current amplification factor.

In FIG. 7, dependence of reverse leakage current on electron beamirradiation dose with applied voltage of 600 V at 125° C. is shown for adevice of a breakdown voltage of 600 V. As apparent in the figure,reverse leakage current decreases with increase of electron beamirradiation dose. Since irradiation of 2 Mrad abruptly reduces reverseleakage current, favorable electron beam irradiation dose is about 2Mrad or more.

Nevertheless, electron beam irradiation generates lattice defects in thebulk of a device, leading to elevation of on-state voltage. The on-statevoltage is an important characteristic, and preferably is as low aspossible. In FIG. 8, the abscissa represents electron beam irradiationdose in Mrad (1 Mrad=10 kGy) and the ordinate represents on-statevoltage. On-state voltage increases with an increase of electron beamirradiation dose. Irradiation of more than 6 Mrad increases the on-statevoltage rapidly. To control the on-state voltage low, electron beamirradiation dose is preferably about 6 Mrad or smaller.

EXAMPLE 3

FIGS. 9 and 10 show a structure of a third embodiment according to thepresent invention. FIG. 9 is a perspective view of a breakdownwithstanding structure, and FIG. 10 is a sectional view of the breakdownwithstanding structure. FIG. 10( a) is a sectional view of wholebreakdown withstanding structure, while FIGS. 10( b) through 10(d) arepartial sectional views of variations from the portion Y in FIG. 10( a).

N type FZ wafer 34 with resistivity of 80 Ω cm is prepared and a thermaloxide film 2.4 μm thick is formed on the front surface of the wafer. Thethermal oxide film in a scribe region for dividing to chips is removedand exposed. After applying boron glass to the exposed region andremoving the boron glass, driving-in is executed at 1,300° C. for 250hour, so that p+ isolation region 31 is formed in the scribe region.Then, a thermal oxide film is formed, which is selectively etched toexpose areas for forming field limit layers 25. Ion implantation ofboron is conducted with a dose of 2×10¹⁵/cm² and an energy of 100 keV,followed by driving-in at 1,150° C. for 200 minutes. Subsequently,openings for forming an active region are formed in the thermal oxidefilm. A gate oxide film 65 nm thick is formed. A polycrystalline siliconfilm is grown on the gate oxide film and selectively etched to form agate electrode. At this moment, the polycrystalline silicon of a portionof the breakdown withstanding structure is removed. Ion implantation ofboron is executed for forming p+ base layer 24 with a dose of 2×10¹⁴/cm²and an energy of 100 keV, followed by driving-in at 1,150° C. for 120minutes. After selectively forming a resist film, arsenic ions areimplanted with a dose of 2×10¹⁵/cm² and an energy of 45 keV. A BPSG(borophosphosilicate glass) film for layer insulation is grown andportions of the film on the active region and field limit layers 25 areopened. Al-1% Si films are deposited to form emitter electrode 28 andgate electrode 7 in the active region, and field limit electrodes 27 inthe breakdown withstanding structure region. Then, a nitride film or apolyimide film is deposited and etched.

The rear surface side of the wafer is ground to a wafer thickness of 200μm by back grinding. At this stage, p+ isolation region 31 is exposed tothe rear surface and spans from the front surface to the rear surface.The rear surface is etched by a thickness of 20 μm using a hydrofluoricacid-nitric acid mixture to make the rear surface smooth. Thickness ofthe wafer is 180 μm at this stage to form n− drift layer 34 having apredetermined thickness. Then, p+ collector layer 29 is formed in thesimilar manner as in Example 1. Al/Ti/Ni/Au are sequentially depositedon the collector layer to form collector electrode 35. Finally, dicingis conducted in the scribe region. Thus, reverse blocking IGBT chips areproduced.

Isolation region 31 is provided for avoiding exposure to the chip endface of the depletion layer that expands from p+ collector layer 29 andn− drift layer 34 of the FZ wafer when reverse bias voltage is appliedto the reverse blocking IGBT, in which electric potential is higher atemitter electrode 28 than at collector electrode 35.

A breakdown withstanding structure is formed in the front surface regionbetween emitter electrode 28 and p+ isolation region 31. P type fieldlimit layer 25 is formed next to emitter electrode 28. Field limit layer25 is in contact with field limit electrode 27 a. Field limit electrode27 a is formed relatively widely beyond field limit layer 25 extendingtowards the p+ isolation region (in the direction towards periphery ofthe device). Several pairs of field limit layer 25 and field limitelectrode 27 are formed arranged towards isolation region 31. Everyfield limit electrode is at each floating electric potential. Theextending direction of the field limit electrodes in the emitterelectrode side is reversed from the extending direction of the fieldlimit electrodes in the isolation region side at intermediate fieldbuffer region 33 disposed at an intermediate portion of the breakdownwithstanding structure. Field limit electrode 27 b on field limit layers25 in the region between intermediate field buffer region 33 andisolation region 31 is formed relatively widely beyond the field limitlayer and extending in the direction of the emitter electrode side (inthe direction towards inner portion of the device), taking reverse biasinto account. Several pairs of a field limit layer and such a fieldlimit electrode 27 b are formed towards the emitter electrode side. Whena depletion layer expands from the emitter electrode side outward, thedepletion layer tends to expand by an effect of field limit electrodes27 a that are extending outward. On the other hand, field limitelectrodes 27 b that are extending inward tend to hinder the expansionof the depletion layer, abruptly changing the effect on the depletionlayer. As a result, concentration of electric field occurs at a tip ofinnermost field limit electrode 27 b, leading to avalanche.

To avoid this electric field concentration and for the depletion layerto smoothly expand towards the side of the field limit electrodes thathave a protrusion in the reversed direction, intermediate field bufferregion 33 is provided for mitigating the electric field concentration.

FIG. 10( b) shows an example without an electrode film like field limitelectrode 27. FIG. 10( c) shows an example in which intermediate fieldbuffer region 33 is replaced by field limit layers 25 a, 25 a and fieldlimit electrodes on the field limit layers are extended and joinedtogether to form electrode 27 d. FIG. 10( d) shows an example in whichthe width of intermediate field buffer region 33 is enlarged and fieldlimit electrodes 27 e, 27 e extending in opposing directions areprovided on the enlarged intermediate field buffer region.

The outermost field limit electrode in contact with p+ isolation region31 is equivalent to a channel stopper electrode disposed in a peripheralportion of an usual IGBT in the forward blocking condition, and thus,called a channel stopper electrode in this specification, too.

FIGS. 11( a) and (b) are sectional views of the breakdown withstandingstructure illustrating development of a depletion layer in a forwardbiased condition (FIG. 11 (a)) and a reverse biased condition (FIG. 11(b)). As shown by a dotted line and an arrow in FIG. 11( a) for theforward biased condition, when an applied voltage is about one tenth ofa breakdown voltage, depletion layer 36 expands from emitter electrode28 towards isolation region 31 up to around the middle position of thebreakdown withstanding structure. In this stage, the extension of fieldlimit electrode 27 a is in a direction to assist expansion of thedepletion layer. Thus, electric field intensity neighboring the pnjunction of field limit layer 25 is mitigated. As the applied forwardbias voltage increases, the depletion layer expands beyond intermediatefield buffer region 33 around middle position of the breakdownwithstanding structure towards isolation region 31. In this stage, theextension of field limit electrode 27 b is in a direction to hinderexpansion of the depletion layer. Thus, with increase of appliedvoltage, development of the front of depletion layer 36 slows down andfinally stops before reaching isolation region 31.

In a reverse biased condition, as shown by a dotted line and an arrow inFIG. 11( b), when an applied voltage is about one tenth of a breakdownvoltage, depletion layer 36 expands from isolation region 31 towardsemitter electrode 28 up to around the middle position of the breakdownwithstanding structure. In this stage, the extension of field limitelectrode 27 b is in a direction to assist expansion of the depletionlayer. Thus, electric field intensity neighboring the pn junction offield limit layer 25 in the isolation region side of the breakdownwithstanding structure is mitigated. As the applied reverse voltageincreases, the depletion layer expands beyond the intermediate portionof the breakdown withstanding structure towards emitter electrode 28. Inthis stage, the extension of field limit electrode 27 a is in adirection to hinder expansion of the depletion layer. Thus, withincrease of applied voltage, development of the front of depletion layer36 slows down and finally stops before reaching the active region.

In a reverse biased condition, however, the depletion layer develops notonly laterally from isolation region 31, but also vertically from therear surface side. As described previously, as the applied voltageapproaches a breakdown voltage, the n− drift layer tends to lackelectric charges (electrons) for forming a depletion layer, furtherpromoting expansion of depletion layer 36. Therefore, it becomesimportant to design adequate distance between field limit electrodes 27and distance between field limit layers 25. The design of the distanceis described below.

If a distance between the field limit layers is narrower than a width ofa built-in depletion layer that expands from the field limit layer tothe n− drift layer in a condition of equipotential between the emitterelectrode and the collector electrode, there is no non-depleted neutralregion between the field limit layers. If the distance between the fieldlimit layers around the front of the depletion layer is narrow atrelatively high reverse biased applied voltage, the depletion layersjoins together and reaches the emitter layer, which is a reach-throughstate and causes increase in leakage current.

FIGS. 12( a) and (b) illustrate this circumstance. FIGS. 12( a) and (b)are partial sectional views showing expansion of a depletion layerbetween the field limit layers. FIG. 12( a) shows a case in which awidth WG of the oxide film is wide, and FIG. 12( b) shows a case inwhich the width is narrow. In FIGS. 12( a) and (b), symbol 25 indicatesa field limit layer, 26, an oxide film, 34, an n− drift layer, and 36 a,a depletion layer. When p type field limit layer 25 is formed by boronion implantation using a mask of oxide film 26 having a width of WG andfollowed by thermal diffusion to a diffusion depth of Xj, lateraldiffusion proceeds to a distance 0.8 Xj from the end of oxide film 26.Therefore, the distance between the field limit layers is preferablywider than or equal to the sum of 1.6 Xj=0.8 Xj+0.8 Xj and 2 Wbi=Wbi+Wbi, Wbi being a width of a built-in depletion layer at zero appliedvoltage. Formula (18) represents this condition.W _(G)≧1.6Xj+2Wbi  Formula (18),where W_(G) is the width of an oxide film between field limit layers,

Xj is the diffusion depth of the field limit layer, and

Wbi is the width of a built-in depletion layer developing from the fieldlimit layer into n+ drift layer in a condition of equipotential betweenthe emitter electrode and the collector electrode.

FIG. 12( a) shows the case W_(G)≧1.6Xj+2 Wbi, and FIG. 12( b) shows thecase W_(G)<1.6Xj+2 Wbi. The situation can be defined in terms of anotherparameter Wg, a distance between the field limit layer; namely Wg≧2 Wbi.

FIG. 13 is a characteristic chart illustrating relationship betweenreverse breakdown voltage and a sum of distances L_(Ni) that is a widthof a neutral region (without joining of built-in depletion layers)between the field limit layers at a zero bias condition. Here,L _(Ni) =W _(G) i−(1.6Xj+2Wbi),where i is an order number of the field limit layer, and

W_(G)i is width of an insulator film of oxide between (i-1)-th and i-thfield limit layer.

It has been discovered that the reverse breakdown voltage rapidlydecreases from an ideal reverse breakdown voltage of a planar junctionwhen the sum becomes smaller than the n− drift layer thicknessW_(drift). As described above, it is important that built-in depletionlayers are not joined together between the field limit layers, so that aneutral region L_(Ni) remains. As applied voltage increases, the neutralregion gradually changes to a depletion layer, and at the same time, adepletion layer expands vertically under the active region from a pnjunction in the rear surface side towards the front surface side. If thesum of the neutral regions in the breakdown withstanding structure in azero bias condition is smaller than the n− drift layer thickness in thevertical direction, a depletion layer in the breakdown withstandingstructure reaches the emitter electrode at an applied voltage lower thana voltage at which a vertically expanding depletion layer reaches theemitter electrode, that is, reach-through occurs and a breakdown voltagedecreases. Therefore, the condition of the following Formula (19) ispreferable.

$\begin{matrix}{{\sum\limits_{i = 1}^{n}L_{Ni}} \geq W_{drift}} & {{Formula}\mspace{20mu}(19)}\end{matrix}$where

$\sum\limits_{i = \bullet}^{n}L_{Ni}$is the sum of the width of the neutral region at zero bias in thebreakdown withstanding structure andL _(Ni) =W _(Gi)−(1.6X _(j)+2W _(bi))where

i is an order number of the field limit layer,

W_(G)i is the width of an insulator film of oxide between (i-1)-th andi-th field limit layer, and

n is the total number of the field limit layers.

EXAMPLE 4

FIG. 14 is a partial sectional view showing a width Lop of an openingbetween field limit electrode 27 a and adjacent field limit layer 25.Passivation layer 37 is provided on the surface of the device. In ahumid environment, negative ions eventually invade into a portionwithout field limit electrode 27 a of a surface region of oxide film 26.The invasion of the negative ions induces positive charges on driftlayer 34 beneath oxide film 26, generating inhomogeneity of potentialdistribution and decreasing breakdown voltage. Accordingly, a simulationhas been made considering the invasion of negative ions with variedwidth Lop of the opening. The results are given in Table 1.

TABLE 1 FBV [V] RBV [V] −10¹²/cm² × q −10¹²/cm² × q Σ edge (elementary(elementary sample W_(Gi) L_(Ni) L_(OP) ΣL_(OP)/ length charge of chargeof number [μm] [μm] [μm] ΣL_(Ni) [μm] 0 electron) 0 electron) sample 1354 237 200.2 0.845 552 1402 806 1312 871 sample 2 390 273 205.0 0.751567 1426 981 1321 1022 sample 3 765 290 177.6 0.612 1302 1450 1433 13561317 sample 4 554 284 111.0 0.391 884 1447 1360 1334 1308 sample 5 509275 96.2 0.350 793 1435 1305 1308 1296

Table 1 shows the result of the simulation for a 1,200 V reverseblocking IGBT. The simulation estimates forward breakdown voltage FBVand reverse breakdown voltage RBV for varied dimensions in the breakdownwithstanding structure. In Table 1, Σ W_(G)i is a sum of the widthsW_(G)i of oxide film 26, Σ L_(Ni) is a sum of widths L_(Ni) of theneutral regions in the breakdown withstanding structure in a zero biascondition, Σ Lop is a sum of distances of openings Lop between an end ofa field limit electrode and an end of an adjacent field limit layer, andthe edge length is a total length of the breakdown withstandingstructure that is a distance from the inner end of the innermost oxidefilm to the outer end of the outermost oxide film. Σ Lop/Σ L_(Ni) is anopening proportion of the field limit electrodes with respect to totalwidths of neutral regions. The simulation showed that forward breakdownvoltage and reverse breakdown voltage for every sample are higher than1,300 V, as shown in columns indicated with “0” for FBV and RBV inTable 1. On the other hand, a simulation assuming existence of negativecharges with a density of 1×10¹²/cm² on the neutral regions having widthof L_(Ni) has demonstrated substantial decrease in both forward andreverse breakdown voltages for Samples 1 and 2. Therefore, Σ Lop/ΣL_(Ni) is preferably less than 0.7.

FIG. 15( a) through (d) are schematic partial sectional views of acomparative example illustrating reach-through of a depletion layer to aprincipal pn junction in the emitter side when the emitter—collectorvoltage is RBV 871V for the sample in Table 1. FIG. 15( a) shows netdoping, FIG. 15( b) shows electron concentration, FIG. 15( c) showsequipotential lines, and FIG. 15( d) shows hole current density. InFIGS. 15( a) through (d), an outer end region of the emitter electrodeis shown in the left side of the figures. The figures only show aportion near the emitter electrode, and do not show a portion near theisolation region. As shown with electron concentration in FIG. 15( b),the charge neutral zone in which the electron concentration isapproximately 6×10¹³/cm³ does not exist in the right-hand side of the Pemitter layer whose x− coordinate is 5 μm or more but under P emitterlayer whose x− coordinate is from ⁻40 to 0 μm. This means the depletionlayer is in a reach-through state to the principal pn junction in thefront surface of the emitter side. As shown with hole current density inFIG. 15( d), reverse leakage current flows in the portion of planartermination edge structure. The circumstances shown in FIGS. 15( a)through (d) correspond to Sample 1 in Table 1.

FIG. 16( a) through (d) correspond to Samples 3 through 5, and areschematic partial sectional views of the breakdown withstandingstructure region with an opening width Lop of 7 μm. FIGS. 16( a), (b),(c), and (d) show net doping, electron concentration, equipotentiallines, and hole current density, respectively. FIGS. 16( a) through (d),like FIGS. 15( a) through (d), show only a portion near the emitterelectrode, and do not show a portion near the isolation region. As shownwith electron concentration in FIG. 16( b), the depletion layer isremote laterally from the principal pn junction in the emitter side withenough distance. As shown with the hole current density in FIG. 16( d),the leakage current flows vertically in the active region under theemitter electrode, demonstrating a stable characteristic.

EXAMPLE 5

FIGS. 17 through 23 illustrate Example 5, in which n type highconcentration layers 38 are provided in the breakdown withstandingstructure region. Impurity concentration in the high concentrationlayers is higher than in the n− drift layer and lower than in the n+emitter region. The n type high concentration layers further prevent adepletion layer from expansion in a reverse bias condition. The highconcentration layer can be formed, for example, by phosphor ionimplantation around the neutral regions in the breakdown withstandingstructure with a dose of 1×10¹²/cm² and an acceleration voltage of 45keV, followed by driving-in at 1,150° C. for 5 hours.

FIGS. 17( a) and (b) are sectional views of an example of embodiment inwhich the n type high concentration layers are formed in both theemitter electrode side and the isolation region side in the breakdownwithstanding structure region. FIG. 17( a) shows a cross section of thebreakdown withstanding structure region, and FIG. 17( b) shows apartially enlarged section of the emitter electrode side. In thisexample, three high concentration layers 38 a are formed in the emitterelectrode side (one between the principal emitter pn junction and firstfield limit layer 25, and two outside the first field limit layer); andthree high concentration layers 38 b are formed in the isolation regionside (one between the isolation region and the outermost field limitlayer 25, and two inside the outermost field limit layer). The highconcentration layers are formed without overlapping with a field limitlayer. The high concentration layers 38 a, 38 b enhance reversebreakdown voltage, reduce reverse bias leakage current, and controldecrease in forward breakdown voltage within about 5%. Symbol 37indicates a passivation layer for protecting the surface of the device.The n type high concentration layers may be formed every space betweenthe field limit layers.

FIGS. 18 through 20 are sectional views of an example of embodiment inwhich an n type high concentration layer is formed in the isolationregion side of the breakdown withstanding structure region. FIG. 18 is aschematic sectional view of the breakdown withstanding structure region.FIG. 19 is a detailed sectional view of the breakdown withstandingstructure region before forming the n type high concentration layers.The numerical values between the dotted lines in FIG. 19 correspond tothe width between the dotted lines in the illustrated embodiment. Thenumerical value is given in μm and “gr0”, “gr7” and “gr13” are thenumbers of guard rings, which are numbered from the side of the activeregion to the side of the dicing line. FIG. 20 shows schematically apartial cross section of the breakdown withstanding structure region.FIGS. 20( a) through (d) illustrate net doping, electron concentration,equipotential lines, and hole current density, respectively. FIG. 19only is drawn reversed in left and right sides, namely, active region inthe right side and isolation region in the left side of the figure. Asshown in FIG. 18, n type high concentration layer 38 c is formed in aregion spanning from a place under channel stopper electrode 21 incontact with the isolation region to intermediate field buffer region33. High concentration layer 38 c suppresses expansion of a depletionlayer in a reverse bias condition in a voltage range of zero to half thebreakdown voltage (up to about 600 V). As a result, reach-through ofdepletion layer to an emitter principal junction is suppressed incomparison with a structure without high concentration layer 38 c, andreverse breakdown voltage is enhanced by 100 V from 1,250 V to 1,350 V.Forward breakdown voltage is not affected because a depletion layer inthe process of expansion encounters no high concentration layer like 38c in the side of the emitter principal junction.

FIG. 21 shows dependence of reverse breakdown voltage on phosphor doseto the high concentration layer of the structure of FIG. 18.Concentration at the surface corresponding to the dose is also written.A dose of more than 1×10¹² atoms/cm² (or a surface concentration of morethan 1×10¹⁷ atoms/cm³) deteriorates reverse breakdown voltage. Thereason for this deterioration is excessive suppression to the depletionlayer in a reverse bias condition, which causes high electric fieldintensity in the portion of the high concentration layer and the p typefield limit layers. Therefore, a phosphor dose is preferably less than1×10¹² atoms/cm², which corresponds to a surface concentration of 1×10¹⁷atoms/cm³.

A similar effect can be obtained by an n type high concentration layer38 d formed in about half the emitter electrode side of the breakdownwithstanding structure region as shown in FIG. 22. High concentrationlayer 38 d is formed by phosphor ion implantation with a dose of lessthan 1×10¹² atoms/cm². The present invention provides a breakdownwithstanding structure that avoids reach-through of a depletion layereven in a reverse bias condition. As shown by these examples, a place toform the high concentration layer is determined depending on selectionof higher breakdown voltage from forward and reverse breakdown voltages.

FIG. 23 is a characteristic chart showing variation of reverse breakdownvoltage in a long term in a THB test (temperature humidity biased test).In the THB test, reverse blocking IGBT chips are mounted in a two-in-onemodule, in which two chips are series-connected to form one module. Areverse bias voltage of 960 V is applied to the module with highervoltage at an emitter electrode than at a collector electrode of a lowerarm IGBT chip. The module is placed in an environment of 85% RH and 125°C. A conventional device having a breakdown withstanding structure of aresistive film begins to decrease breakdown voltage at 1,000 hours. Incontrast, a device of the invention demonstrates stable reversebreakdown voltage after over 3,000 hours, even at 5,000 hours. Thepresent invention provides a breakdown withstanding structure todemonstrate stable reverse blocking capability in long term reliabilitytest. The present invention thus provides a reverse blocking IGBT thatenables a matrix converter without a series-connected diode.

A reverse blocking semiconductor and a method for its manufacture havebeen described according to the present invention. Many modificationsand variations may be made to the techniques and structures describedand illustrated herein without departing from the spirit and scope ofthe invention. Accordingly, it should be understood that the methods anddevices described herein are illustrative only and are not limiting uponthe scope of the invention.

1. A reverse blocking semiconductor device comprising: a drift layer ofa first conductivity type; a MOS gate structure including a base layerof a second conductivity type selectively formed in a front surfaceregion of the drift layer, an emitter region of the first conductivitytype selectively formed in a surface region of the base layer, a gateinsulation film covering a surface area of the base layer between theemitter region and the drift layer, and a gate electrode deposited onthe gate insulation film; an emitter electrode in contact with both theemitter region and the base layer of the MOS gate structure; anisolation region of the second conductivity type surrounding the MOSgate structure through the drift layer and extending across the entirethickness of the drift layer; a collector layer of the secondconductivity type formed on a rear surface of the drift layer andconnecting to a rear side of the isolation region; a collector electrodein contact with the collector layer; a plurality of field limit layersof the second conductivity type in the front surface region of the driftlayer between the emitter electrode and the isolation region, each ofthe field limit layers having a ring shape; and a plurality of fieldlimit electrodes, each in contact with each of the field limit layers,each of the field limit electrodes having a ring shape, and being at afloating electric potential; wherein a plurality of the field limitelectrodes exist in the side of the emitter electrode and each of thefield limit electrodes in the side of the emitter electrode has a largeroutward extension portion than an inward extension portion; and aplurality of the field limit electrodes exist in the side of theisolation region and each of the field limit electrodes in the side ofthe isolation region has a larger inward extension portion than anoutward extension portion.
 2. The reverse blocking semiconductor deviceaccording to claim 1, additionally comprising at least one highconcentration layer of the first conductivity type in at least a portionof one of the front surface region of the drift layer between theemitter electrode and the isolation region, the high concentration layercontaining a higher concentration of impurities than the drift layer. 3.The reverse blocking semiconductor device according to claim 2, whereina surface concentration of impurities in the high concentration layer isless than 10¹⁷ cm⁻³.
 4. The reverse blocking semiconductor deviceaccording to claim 1, wherein a distance Wg between the adjacent fieldlimit layers is larger than 2 times Wbi, where Wbi is a width of abuilt-in depletion layer extending from the field limit layer towardsthe drift layer in a condition in which the emitter electrode and thecollector electrode are at an equal electric potential.
 5. The reverseblocking semiconductor device according to claim 1, whereinW _(G) i>1.6Xj+2Wbi in which W_(G)i is a width of an insulator filmbetween (i-1)-th field limit layer and i-th field limit layer, Xj is adiffusion depth of the field limit layer, and Wbi is a width of abuilt-in depletion layer extending from the field limit layer towardsthe drift layer in a condition in which the emitter electrode and thecollector electrode are at an equal electric potential.
 6. The reverseblocking semiconductor device according to claim 1, wherein a thicknessof the drift layer W_(drift) satisfies the inequality in the followingformula (2): $\begin{matrix}{{\sum\limits_{i = 1}^{n}L_{Ni}} \geq W_{drift}} & {{Formula}\mspace{20mu}(2)}\end{matrix}$ whereinL _(Ni) =W _(Gi)−(1.6X _(j)+2W _(bi)) i is an order number of the fieldlimit layer, W_(G)i is a width of an insulator film of oxide between(i-1)-th and i-th field limit layer, n is the total number of the fieldlimit layers, Xj is a diffusion depth of the field limit layer, and Wbiis a width of a built-in depletion layer extending from the field limitlayer towards the drift layer in a condition in which the emitterelectrode and the collector electrode is at an equal electric potential.7. The reverse blocking semiconductor device according to claim 6,wherein the sum Σ L_(N)i and a sum Σ L_(OP)i satisfies an inequality ΣL_(OP)i/Σ L_(N)i<0.7, in which L_(OP)i is a distance between (i-1)-thfield limit electrode and i-th field limit layer.
 8. The reverseblocking semiconductor device according to claim 1, additionallycomprising an intermediate field buffer region of the secondconductivity type in a surface region of the drift layer between theplurality of field limit electrodes in the side of the emitter electrodeand the plurality of field limit electrodes in the side of the isolationregion.